Methods of recovery of FM signals using direct conversion receivers are known. Such methods typically involve translation of a signal received on a carrier frequency by a receiver (FIG. 1) from a selected channel to an intermediated frequency (IF), conversion of the IF signal into a square wave and direct conversion to recovered audio by application of the square wave, and a delayed version of the square wave, to a quadrature detector (e.g., an exclusive-or gate).
Such a receiver (FIG. 1) could be used within a cellular communication system operating within the 800-900 MHz transmission band decoding signals with channel spacing of 25 kHz. Within such a system a fully modulated FM signal would deviate from a center point of a carrier frequency of the channel by plus or minus 10 kHz. The rate with which the signal deviates between plus to minus 10 kHz (for a fully modulated signal) is representative of (and tracks) the originally encoded audio signal (e.g., for an input audio signal of 2 kHz the FM signal would cycle between plus and minus 10 kHz of the carrier frequency at a rate of 2 kHz).
Frequency translation of the FM signal from the channel frequency to the IF frequency is typically accomplished by first translating the FM signal to a base band frequency (zero-RF) and then upconverting to the IF frequency. Translation from the channel frequency to the zero-RF state is accomplished by mixing the received signal with an output of a first local oscillator (LOCAL OSC #1). Translation of the FM signal to a zero-RF state, on the other hand, carries with the FM signal, amplitude variations associated with the transmission channel. Amplitude variations of the FM signal, as is known to those in the art, are decoded by the detector as additional audio information. The additional audio information represents interference which degrades the quality of the detected signal.
One way to eliminate amplitude variations is to convert the translated FM signal (at an IF frequency) into a square wave by clipping. The square wave and a delayed copy of the square wave are then input to the exclusive-or gate. The output of the exclusive-or gate is a series of pulses, the average of which is representative of a transmitted audio signal.
The received FM signal (at the carrier frequency) is converted into a square wave by first mixing (in a first mixer) the received signal with a first output of a first local oscillator (LOCAL OSC #1) and, then, mixing within a second mixer the received signal with a second output of the first local oscillator in quadrature relationship with the first output. The outputs of the two mixers (FIG. 4) are quadrature components of the FM signal at zero-RF.
The quadrature components of the FM signal at zero-RF, on the other hand, cannot be summed and clipped at base band frequencies (for application to the exclusive-or gate) because harmonics of audio information (e.g., audio information at 2 kHz would have harmonics at 4, 8 or 16 kHz) would still fall within the bandwidth of the received signal. Because of the problem of harmonics, the quadrature components of the FM signal at zero-RF are mixed, in a second set of mixers, with quadrature components of a second local oscillator (LOCAL OSC #2) to the intermediate frequency (e.g., at 131 kHz) and summed before clipping. The output of the clipper is a square wave from which the audio information may be recovered by application to the exclusive-or gate.
The information is recovered from the square wave by applying the square wave, and a delayed version of the square wave to the exclusive-or gate. The time delay is chosen to place the delayed square wave 90 degrees behind the undelayed squarewave at the center frequency of the IF. At the center frequency of the IF, the output pulses of the exclusive-or gate are the same width as the spaces between the pulses. The average output of the exclusive-or gate at the center frequency of the IF is, consequently, one-half the voltage of the output pulse of the exclusive-or gate. Where the IF deviates upwards in frequency with the FM signal (towards 131 kHz plus 10 kHz) the average output of the exclusive-or gate also increases towards the voltage of a full scale pulse. Where the IF deviates downward (131 kHz minus 10 kHz) the average output of the exclusive-or gate declines towards zero. Because of the relationship of the output of the exclusive-or gate to the time delay, the time delay value of the delayed square wave must be carefully chosen (calibrated) to avoid clipping of the output audio signal.
While recovery of FM signals by direct conversion works well, the reliability of such a system depends on the average output of the exclusive-or gate. In order for the output of the detector to faithfully reproduce the input audio signal the average output of the exclusive-or gate must remain centered. Since the detector is an on-off device the average output of the detector is determined by the relationship of the square wave and the delayed square wave. Where the temporal relationship of the delayed square wave changes (because of aging, temperature, etc.) the detector becomes off-centered, resulting in clipping of the output signal.
Delay of the square wave is typically accomplished through use of a bandpass filter operating at a center point of a filtering (attenuation) curve. Operation at the center point provides sufficient phase shift to provide the delay desired at the detector. Where the operating point of the filter shifts from the center point because of IF frequency shifts, or filter component parametric changes, the reliable operation of the detector is adversely effected. Because of the importance of FM conversion through direct detection techniques, a need exists for a more reliable method of delaying the square wave signal in advance of detection.